This invention relates to a method of producing tin-free steel sheets having improved adhesion between metallic chromium and hydrated chromium oxide coatings and between the hydrated chromium oxide coating and a paint film, and exhibiting improved resistance to a retorting treatment.
One of the prior art methods for producing tin-free steel sheets uses two separate baths to deposit metallic chromium in one bath and then deposit a hydrated chromium oxide coating in a separate bath. This method has the benefit of easily controlling the thickness of the respective coatings. Particularly when a chromic acid solution is used as the electrolytic chromate treating bath as disclosed in Japanese Patent Publication No. 47-35172, the amount of hydrated chromium oxides deposited is increased in proportion to the quantity of electrically for electrolysis, enabling accurate control of the deposition amount. However, since the baths used in the double bath method have different compositions, transferring chromium plated steel sheets to an electrolytic chromate treating bath directly from a chromium plating bath introduces the possibility that the anion of a plating assistant which has been codeposited in the hydrated chromium oxide coating during chromium plating is left in the coating of the final product, the chromium plating solution entrained on the steel sheet surface is carried into the electrolytic chromate treating bath to vary the amount of hydrated chromium oxides deposited, and that portion of the codeposited assistant anion which is partially dissolved out is again codeposited. To avoid such undesirable problems, the chromium plated steel sheet should be fully rinsed with water before it is passed into the electrolytic chromate treating bath. However, it has been found that rinsing after chromium plating is insufficient to remove the chromium plating assistant anion which has been codeposited with hydrated chromium oxides during chromium plating. Particularly when chromium plating uses sulfuric acid or its derivatives as the assistant, in spite of the subsequent electrolytic chromating treatment in a chromate bath, the sulfate residues codeposited are left in the final hydrated chromium oxide coating and detract from its adhesion to a paint film.
Making extensive investigations to eliminate the shortcomings of the double bath method of producing tin-free steel sheets, the inventors have found that by subjecting a steel sheet which has been chromium plated in a chromium plating bath to reverse electrolysis in the same bath such that the hydrated chromium oxides formed during chromium plating and the assistant anion codeposited therewith may be decreased, thereafter fully rinsing the chromium plated steel sheet with water and subjecting it to an electrolytic chromate treatment in an aqueous chromic acid solution bath, the retorting resistance of the steel sheet is significantly improved.